1. Field of the Invention
The present invention relates to CMOS image sensor design and more particularly to microlens alignment procedures of CMOS image sensors.
2. Background of the Related Art
There has been an increase of digital image devices using CMOS image sensors. A conventional CMOS image sensor requires a matching imaging lens to have certain ray angle incident on its sensor surface to generate acceptable image data output. In an effort to mitigate the ray angle requirement, the CMOS sensor may be customized to accept incident rays at large angles, especially for the pixels at corners and edges thereof.
One common practice of CMOS sensor customization is shifting a microlens array of the sensor to match the incident rays at large angles. However, in the application of the shifting technique, the non-symmetric nature of the CMOS sensor pixel layout may create non-symmetric brightness distribution over the image output, where the non-symmetric nature may be more pronounced at the corners and edges of the image output. In addition, such non-symmetric brightness may be accompanied by improper color balance, i.e., the color of the image of a white light source is not white over the entire image output.
To resolve the appearance of non-symmetric brightness distribution over the image output, the existing approaches have attempted several symmetric layouts for each pixel of the CMOS sensor. Such approaches may impose many limitations and restrictions to a layout designer and some tradeoff may be necessary to accommodate the symmetric layout with additional silicon real estate. In addition, the entire CMOS pixel layout may be modified upon unsatisfactory image output, which requires lengthy and expensive turn-around processes. Furthermore, the existing approaches cannot correct the non-symmetric nature that may be rooted in other sources, such as chemical contamination occurred during the CMOS process for producing a silicon die, electrical field generated by metal layers of the CMOS sensor, imperfect masks used in the CMOS process and other unknown sources. Thus, there is a need for an improved method for resolving non-symmetric brightness distribution.